In most of the chemical treatment plants, cooling water systems such as heat exchangers, piping and cooling tower forms an integral part of the control process operations. For any control process, the heat transfer performance of the cooling tower must be maintained. Water in cooling towers needs to be treated to control microbial growth, scale formation, and metal corrosion of the process equipment surfaces. Scaling is the formation of crystalline deposits of water soluble salts such as calcium carbonate, magnesium hydroxide and calcium sulphate on piping, heat exchangers and other process equipment surfaces. Scaling occurs when the highly soluble and naturally occurring calcium bicarbonate decomposes into calcium carbonate and CO2 gas. Unlike calcium bicarbonate, calcium carbonate has a very low solubility in water, approximately 15 mg/L, and unlike most compounds it tends to precipitate out of solution with increasing temperature. At the air-water interface of a cooling water system, a portion of water evaporates leaving dissolved solids behind and increasing the remaining concentration of total dissolved solids (TDS) such as calcium carbonate. The scale acts as an insulator and thereby reduces cooling tower efficiency resulting in an increased backpressure, higher pumping requirements, and increased energy use. In heat exchangers such as plate heat exchangers a thin layer of scale can reduce heat exchange efficiency by as much as 0.15%. Scale build-up also causes bio-fouling in a reactor. Bio-fouling has been recognized as an important contributor to impaired heat transfer causing decrease in thermal efficiency and increased power consumption. Preventing scale build-up is one of the primary objectives of a traditional cooling tower chemical treatment program.
Cavitation method is often employed at the cooling tower water circuit to take care of bio-fouling and water scaling. Cavitation is the formation, growth, and implosion of vapour bubbles in a liquid. It can be created by sound waves (ultrasonic or acoustic cavitation), lasers, or by fluctuations in fluid pressure (hydrodynamic cavitation). Cavitation method can be used to facilitate the precipitation and removal of calcium carbonate in the water. The following equation describes the reaction followed in the cavitation method.

The soluble calcium bicarbonate salts in water form solid calcium carbonate and carbon dioxide gas, and as the carbon dioxide gas is removed, the equilibrium is shifted to the right side of the equation. Carbon dioxide is degassed from water and the solid precipitate is easily removed from water through the use of a cyclonic separator or a filtration system. Because both calcium bicarbonate and calcium carbonate are simultaneously removed from the water stream, the solubility limit of calcium carbonate is not reached and scaling is inhibited. Thus, in hydrodynamic cavitation (HC) chamber, water undergoes cavitation and calcium bicarbonate (CaHCO3)2 from water is precipitated out in the form of calcite (CaCO3). In the chamber, the chemical equilibrium of the carbonate species in water is shifted, driving the reaction equilibrium to the right, precipitating out calcium bicarbonate. The calcium carbonate crystals steadily grow and are easily removed from the water stream using a filtration system.
Hydrodynamic cavitation provides increased nucleation sites in the form of small sized CaCO3 colloids. These colloids then act as growth sites for other dissolved ions. As continued crystal growth is thermodynamically favoured over the formation of new nuclei, the calcite crystals continue to grow in size. Coagulation increases due to greater mass attraction, and the filtration system then removes the larger particles that precipitate out.
Hydrodynamic cavitation involves the process of vaporisation, bubble generation and bubble implosion which occurs in a flowing liquid as a result of a decrease and subsequent increase in pressure. Cavitation will only occur if the pressure declines to some point below the saturated vapor pressure of the liquid and subsequent recovery above the vapor pressure. If the recovery pressure is not above the vapor pressure then flashing is said to have occurred. In pipe systems, cavitation typically occurs either as the result of an increase in the kinetic energy (through an area constriction) or an increase in the pipe elevation.
Hydrodynamic cavitation can be produced by passing a liquid through a constricted channel at a specific velocity or by mechanical rotation of an object through a liquid. In the case of the constricted channel and based on the specific (or unique) geometry of the system, the combination of pressure and kinetic energy can create the hydrodynamic cavitation cavern downstream of the local constriction generating high energy cavitation bubbles.
Hydrodynamic, cavitation systems for water descaling are known in art. Some prior art suggests treating water or fluids using Hydrodynamic cavitation reactors coupled to ultraviolet radiation for improving the efficiency of water treatment process.
U.S. Pat. No. 4,990,260 describes a device for purifying water comprising two separate steps. In the first step, the polluted water is transported through a venturi, arranged in a reactor chamber, such that cavitation is caused. In the second step the oxidizable contaminants are oxidized by UV light in a separate reactor chamber.
US20100090124 discloses a method and apparatus for disinfecting fluids. The method generally includes cavitating and irradiating a fluid by exposing the fluid to an ultraviolet radiation. The patent discloses combining rotating cavitator and UV lamps in a single reactor for improving the transmittance of the circulating fluid. However the patent does not discloses or suggests the process of water descaling.
US20130248429 discloses a method for purifying water in a reactor containing hydrodynamic cavitation coupled to a pulsed/continuous UV radiation. However the patent does not focuses on the process of water descaling.
Hydrodynamic cavitation systems known in the art are quite expensive on the operating cost. An extent of pressure drop across the restriction device translates to the energy cost. It is observed that HC rector with moderate pressure drop in the re-circulatory mode of operation yields prolonged cavitation exposure which improves salt removal efficiency. The same volume of water is subjected to hydrodynamic cavitation over a prolonged time through the desired number of recirculations which increases the energy cost.
Thus it is desirable to provide an apparatus for water descaling which provides effective salt precipitation in a single pass of water circulation and thereby reduce number of water recirculation loops, resulting in reduced energy costs.